1. Field of the Invention
This invention relates to a process for the production of a magnetic recording medium having a ferromagnetic metal thin film as a magnetic recording layer and more particularly, it is concerned with a process for producing a magnetic recording medium excellent in magnetic properties by a vacuum vapor deposition method.
2. Description of the Prior Art
Up to the present time, magnetic recording media of binder type have widely been used in which magnetic oxide powders such as .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, Berthollide compounds of .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, CrO.sub.2, etc. and ferromagnetic alloy powders are dispersed in organic binders such as vinyl chloride--vinyl acetate copolymers, styrene--butadiene copolymers, epoxy resins, polyurethane resins, etc., coated onto non-magnetic supports and dried. Of late, on the other hand, magnetic recording media of the so-called non-binder type have become worthy of notice, in which no binder is used and magnetic recording layers consisting of ferromagnetic metal thin films are formed by the vapor deposition method such as vacuum evaporation, sputtering or ionic-plating, or by the plating method such as electroplating or electroless plating, with the increase of requirements for high density recording, and various efforts have been made for the purpose of realizing the magnetic recording media of this type.
Since in the prior art magnetic recording media of binder type, metal oxides having a smaller saturation magnetization than ferromagnetic metals are mainly used as a magnetic material, the thickness reduction required for high density recording has reached a limit because of resulting in lowering of the signal output and complicated processings and large special installations to recover the solvent and to prevent the public hazards are required for the production of such magnetic recording media. On the contrary, the magnetic recording media of non-binder type have the advantages that the process for the production thereof is simple and an extreme thickness reduction is possible because a ferromagnetic metal having a larger saturation magnetization than metal oxides is formed as a thin film under such a state as being free from non-magnetic materials such as organic binders.
In particular, the vacuum vapor deposition method has hitherto been considered important as a method for producing a magnetic recording medium of non-binder type because of obtaining a large deposition speed of film with a simple production step and without a step of treating a waste liquor as in the case of plating. The following methods are known as a method of obtaining a ferromagnetic thin film having a high coercive force and high squareness ratio required for a magnetic recording medium by vacuum vapor deposition:
1. A method comprising controlling the degree of vacuum or vapor deposition speed (A. V. Davies et al, "IEEE Trans. Magnetics", Vol. MAG-1, No. 4 (1965), page 344; U.S. Pat. No. 3,787,237),
2. A method comprising selecting the compositions of vapor deposited metals (Z. Piskacek et al, "Thin Solid Films", Vol. 2 (1968), page 487; Japanese Patent Publication No. 23025/1968, Japanese Patent Application (OPI) No. 105508/1974; U.S. Pat. Nos. 3,516,860 and 3,775,179),
3. A method comprising making an evaporated beam of ferromagnetic material incoming slantly to a substrate (W. J. Schule, "J. Appl. Physics", Vol. 35 (1964), page 2558; Japanese Patent Publication Nos. 29944/1964, 19389/1966 and 13912/1967; Japanese Patent Application (OPI) No. 74911/1974, and U.S. Pat. Nos. 3,342,632 and 3,342,633), and
4. A method comprising forming a vapor deposited oxide film in an oxidizing atmosphere (J. W. Schneider et al, "IEEE Trans. Magnetics", Vol. MAG-9, No. 3 (1973), page 183; Japanese Patent Application (OPI) Nos. 37695/1973 and 73103/1974; U.S. Pat. No. 3,700,500).
These methods, however, are unsatisfactory because of the disadvantages that desired magnetic properties cannot sufficiently be obtained, the vapor deposition step is complicated, the adhesion between the magnetic thin film and substrate is not good and the vapor deposition efficiency is low.